Raw vegetable oils obtained from crushing vegetable seeds require considerable processing to provide commercially acceptable products.
The standard processing steps for treatment of vegetable oils are outlined in many reference journals, articles and textbooks. Examples of such treatments are found in "Rape Seed Chemistry and Technology", Elsevier Science Publishers 1990, H. Niewiadomski: "High and Low Erucic Acid Rapeseed Oils", Kramer et al., Academic Press 1983; and Bailey's Industrial Oil and Fat Products, Volume 2, 4th Edition, John Wiley & sons.
The standard steps in the treatment of raw vegetable oils include:
i) degumming of the raw oil to remove by separation hydrophyllic materials which include phospholipids,
ii) optionally, phosphoric acid pretreatment of degummed oils may be carried out to remove remaining phospholipids with the removal of at least some chlorophyll color impurities, such precipitate being removed from the treated oils before the next step to avoid the resolubilization of precipitates,
iii) neutralizing the oil with a neutralizing agent and separating the developed soaps from the neutralized oil,
iv) optionally, water washing the oil to remove the residual soaps developed during neutralization,
v) treating the neutralized oil with bleaching clay to remove the remaining chlorophyll color impurities as well as other residual material remaining from neutralization such as soaps, phospholipids and carotenoid compounds, and
vi) deodorizing the bleached oil to provide an oil having a commercially acceptable flavor and color.
A process for treating vegetable oils is marketed under the trade-mark ZENITH PROCESS. It is available from Campro Agra Limited, Mississauga, Canada. The ZENITH PROCESS comprises the steps of degumming an oil by use of phosphoric acid, separating the phosphoric acid sludge from the oil, neutralizing the free fatty acids in the oil, removing the developed precipitates including soap stock from the oil and bleaching the oil to remove any remaining chlorophyll color impurities. The process includes a final filtration step and, optionally, a deodorization step. In the ZENITH PROCESS and many other commercially available processes, the degumming step is conducted in the presence of an acid to assist in removal of the non-hydratable forms of phospholipids. It is now fairly common to use citric acid or malic acid as a degumming agent. Phosphoric acid has also been suggested for this purpose. The acid is thought to convert non-hydratable phospholipids to hydratable forms which can then be readily removed from an oil. In the ZENITH PROCESS, such treatment of an oil with phosphoric acid is carried out at temperatures in the range of 35.degree. C. to 50.degree. C. with no control of the moisture content in the oil.
Raw vegetables oils, depending upon their species, contain varying amounts of chlorophyll. It is generally recognized that oil from rapeseed and particular varieties thereof, namely canola, contain very high levels of chlorophyll; frequently, 10 to 30 ppm.
The presence of chlorophyll renders a raw oil a dark black/green color. Such oils also contain high levels of phospholipids commonly called, "gums". Canola and other raw vegetable oils are generally destined for commercial use where a light color is important; therefore, the chlorophyll color impurities have to be removed from the oils before processing into final commercial products.
Raw oil can be processed in accordance with conventional techniques for degumming an oil. Degumming is generally carried out by water treatment in which the hydrophyllic precipitates are removed by centrifugation. Commonly, an acid is used to enhance the removal of non-hydratable forms of phospholipids. Citric acid or malic acid is frequently used in oil degumming processes. When acid is used in deguming a raw oil the degumed oil is commonly referred to as, "super-degummed vegetable oil".
The chlorophyll color impurities are due to the presence of chlorophyll and its derivatives. For purposes of the discussion of this invention, it is understood that wherever the term chlorophyll is used it is intended to also include chlorophyll derivatives and degradation products. These compounds are oxidatively unstable. Their removal enhances the stability of an oil as well as its appearance.
The conventional method to remove chlorophyll color impurities is to use bleaching clay. This clay absorbs the chlorophyll color impurities thereby removing them for the oil. The high chlorophyll content of rapeseed and canola oils requires bleaching clay levels two to three times greater than that needed for other vegetable oils. A method for removing chlorophyll color impurities in canola oil either without the use of bleaching clay or with reduced amounts of clay is desirable so as to reduce both clay costs and oil losses and increase system throughout. Bleaching is often the rate limiting step for processing canola oil.
Investigations concerning the use of phosphoric acid as a degumming agent for vegetable oils and in particular rapeseed oil as well as use of phosphoric acid in the removal of chlorophyll color impurities from rapeseed oil have been reported by Helena Szemraj of the Institute of Fat and Oil Industry, Warsaw, in the papers entitled, "Review of Technological Method for Degumming Vegetable Oils" (1973), "Phosphoric Acid as a Deguming Agent for Rape Seed Oil" (1973) and "Effect of Phosphoric Acid on the Color Change in Rape Seed Oil" (1974). A process is described for reducing chlorophyll color impurities in the oil by treating degummed rapeseed oil having a moisture content not exceeding 0.05% by weight and treating the oils at temperatures in the range of 60.degree. C. to 65.degree. C.. The quantity of phosphoric acid used is in the range of 0.05% through 0.3% by weight of a 75% aqueous solution of phosphoric acid, H.sub.3 PO.sub.4. In this treatment a precipitate is formed which contains the chlorophyll color impurities. The precipitate is then removed from the oil by filtration using filter paper. This treatment with phosphoric acid reduced the chlorophyll color impurities in the oil. The reduction of the amount of chlorophyll color impurities was determined by spectrophotometric analysis of the oils performed in the visible range of the spectrum at a wavelength of 400 to 750 nm. Additions of 0.05% of 75% H.sub.3 PO.sub.4 reduced chlorophyll color impurities in the oils by 50% to 70%. Use of 0.1% by weight of 75% H.sub.3 PO.sub.4 reduced chlorophyll color impurities in the oil in the range of 60 to 90%. Mixing times in the range of 15 min. were required to effect the necessary precipitation of chlorophyll color impurities from the oil.
The practice described by Szemraj and Bergmann et al., Fette Seifen Anstrichmittel Vol. 66, No. 3, 1964 p. 203-206, "A New Method for Refining Edible Oils and Fats--The Zenith Process", includes removing the precipitate formed by the phosphoric acid treatment before the step of neutralizing the oil because neutralization will re-solubilize the precipitate and cause the chlorophyll color impurities to go back into solution. Hence, a significant drawback of these two techniques is the requirement of an intermediate separation step via filtration or centrifugation prior to neutralization. In a commercial practice, this results in a significant loss of oil plus increased costs of processing.